Silicon controlled rectifiers (SCR) are devices capable of conducting a high current at a low operating voltage. The voltage across the SCR at which it sustains current flow is referred to as the holding voltage. SCRs are often used for electrostatic discharge protection (ESD) in semiconductors. For a classical SCR the holding voltage is typically around 1.2V. When an SCR is triggered by an ESD event, it conducts current from the ESD event safely, protecting the semiconductor device.
When an SCR is implemented in a system operating with a power supply voltage that is higher than 1.2V, the SCR may become latched-up. A latched-up SCR conducts current during normal operation and not just during an ESD event as a protection device. As such, the SCR may often burn out.
To solve the latch-up problem so that SCRs may be implemented in systems operating with higher power supply voltages, the holding voltage of the SCR may be increased by adding diodes in series with the SCR or by stacking multiple SCRs in series. The problem with these solutions in high voltage (HV) applications is that too many additional elements are necessary to reach a holding voltage above the supply voltage. A diode may only increase the holding voltage by 0.7 V which will require too many diodes to be practical. Likewise, it is not practical to stack as many SCRs in series as necessary. Due to the number of additional elements, the area needed to implement the diodes or SCRs is too large. Furthermore, the leakage due to the Darlington effect from the many additional devices will be too high.
Alternatively, in HV technologies other devices may be used such as Zener diodes, ground-gated n-type channel metal oxide semiconductors (GGNMOS), and resistive complementary metal oxide semiconductors (RC-MOS), however these also take up a large area and exhibit triggering problems.
A new device with a high holding voltage above the power supply voltage and a high current capability is needed.